A conventional image processing system includes a charge-coupled device (CCD), an analog front end (AFE) and a digital image processor. The AFE interfaces between an analog device, e.g., CCD, and a digital device and functions to convert the analog data from the analog device to digital data for the digital device.
FIG. 1 is a conceptual block diagram illustrating an image processing system. An image of an object 110 is formed by an image forming element, depicted as a lens 120, onto an analog image detecting device such as a CCD 130. The AFE 140 converts the analog data from the CCD to digital data and forwards the converted digital data to a digital post processor 150, which processes the data to be transmitted to one or more data/display devices, such as computer 170-A, storage disk 170-B, printer/facsimile machine 170-C or camcorder 170-D.
The analog device used for acquiring the analog data for the image can be referred to as a sensor. The sensor can be a contact image sensor (CIS) in which the object is in contact with the system, such as a piece of paper on the glass of a copy machine, facsimile machine or scanner. The CIS can be a CCD CIS or a CMOS CIS.
FIG. 2 is a schematic block diagram of one example of an AFE 140 in an image processing system. The AFE 140 can be regarded as including two parts, namely, the analog signal processing section and the digitizer. The analog signal processing section includes a correlated double sampler (CDS) 141 and a programmable gain amplifier (PGA) 142. The digitizer can include an analog-to-digital converter (ADC) 143. The CDS 141 receives the input analog image signal via an input terminal 144 and samples the input signal. The PGA 142 receives the sampled signals from the CDS 141, amplifies the samples using a variable gain and provides an output signal to the ADC 143. The ADC 143 digitizes the signal from the PGA 142 and provides the digital output at an output terminal 145.
FIG. 3 is a waveform diagram illustrating a typical input analog image signal. With reference to FIG. 3, for each pixel, the signal includes a reset portion, a black portion or reference portion and a signal portion as shown. The real signal or data portion of the signal extends between the dashed lines in the waveform diagram at points labeled 1 and 2.
At points 1 and 2, the CDS 141 samples the data signal. The CDS 141 subtracts the data signal from a reference signal, i.e., the black signal. The difference of the sampled data is a real signal.
With reference to FIG. 3, it is noted that the signal levels, and, in particular, the black reference signal, vary from pixel to pixel. This is due to different characteristics of the diodes in the pixels.